The present invention refers to a fully differential operational amplifier of the folded cascode type.
In applications where it is necessary to have high direct current (xe2x80x9cDCxe2x80x9d) gain without bandwidth restrictions, single stage amplifiers are used with the output transistors cascode connected, and with a feedback loop for each output transistor, to increase the output impedance. These feedback loops are commonly called gain amplifier circuits (gain boosting circuits) because of their function.
A fully differential operational amplifier with a single stage includes four output transistors (two for each output). Therefore, four feedback loops are needed, which normally are realized by means of four unbalanced operational amplifiers. As disclosed in the U.S. Pat. No. 5,748,040, a further realization of the feedback loops is effected by replacing the four individual output operational amplifiers with two differential operational amplifiers. The principal advantages of this realization are a reduced current consumption and the use of a smaller silicon area.
According to the present invention, a reduced current consumption, the use of less silicon area, and other objectives are achieved by means of a fully differential operational amplifier of the folded cascode type comprising: a differential output stage; a differential input stage able to drive said output stage; said differential output stage including a first branch having at least a first and a second transistor and a second branch having at least a third and a fourth transistor; said first and second branches coupled to a first and to a second voltage source; and a feedback circuit of said first, second, third and fourth transistors wherein said feedback circuit is constituted by a single amplifier having four inputs and four outputs, said four inputs taking the voltages present on terminals of said first, second, third and fourth transistors, and said four outputs each providing a voltage to the control elements of said first, second, third and fourth transistors, which said voltage depends on the value of the input voltage of said four inputs.
Such objectives are also achieved by means of an amplifier, having four inputs and four outputs, able to provide an output voltage that depends on the value of the input voltages of said four inputs wherein: said amplifier includes a first differential pair composed of a first and a second transistor having their drains respectively connected to two output terminals and coupled to the first voltage source through two current generators; the gates of said first and second transistors coupled to two input terminals; a second differential pair composed of a third and a fourth transistor having their drains connected respectively to two output terminals and coupled to the second voltage source through two current generators; the gates of said third and fourth transistors coupled to two input terminals; the sources of said first, second, third and fourth transistors connected together; and a voltage generator applied between the first voltage source and said transistor sources.